Field of the Invention
The present invention relates to a permanent magnet synchronous motor and a winding-switching motor driving device, and a refrigeration air conditioner and an electric vehicle using the same.
Description of the Related Art
The permanent magnet synchronous motor (also simply referred to as “motor” below as needed) is characterized by having higher efficiency than the induction motor, and the application range of the motors has been widened to include not only the field of home appliances, but also the fields of industrial apparatuses and electronic motors.
In addition, the motors, appliances, and apparatuses are required to achieve higher efficiency at a low-speed rotation range (light load) in order to meet the movement toward prevention of global warming and energy saving, and are also required to have ability to drive at a high speed rotation range (high load) in order to improve the sense of use (comfortability) of the appliances and apparatuses.
As an example, a room air conditioner as a home appliance is required to improve both an annual energy consumption efficiency (Annual Performance Factor, abbreviated as “APF” below as needed) which is an indicator of energy saving, and a heating capability at an outdoor temperature of 2° C. (heating capability under low temperature), which is an indicator of high output.
As another example, a main engine of an electronic automobile or the like is generally driven at low speed with high torque and at high speed with low torque, and is required to achieve higher efficiency under the aforementioned driving conditions.
As means for making a motor driving device achieve higher efficiency (at a lowspeed rotation range, in particular), there is low speed design of the motor by increasing an amount of magnet and windings. In the motor with low speed design, however, the drive range may be narrowed due to an increase in an induced voltage generated at a high-speed rotation range, and the efficiency at the high-speed rotation range may decrease significantly.
To address this, a method of boosting a DC voltage has been put into practice as means for enlarging the high-speed rotation range of the motor with low speed design. This method, however, requires addition of a circuit for boosting a DC voltage, and has problems of an increase in the circuit scale, and an increase in the loss of the booster circuit or the like.
As means for solving the aforementioned problems, there have been proposed Japanese Patent Application Publication Nos. 2010-200439 (Patent Document 1), 2008-178207 (Patent Document 2), 2008-219956 (Patent Document 3), 2006-136144 (Patent Document 4), and 2013-121222 (Patent Document 5).
Patent Document 1 describes a method of decreasing a generated induced voltage by using a method of switching windings of the motor with a mechanical switch (see [Abstract] and FIG. 1).
Patent Document 2 describes a method of switching a neutral point of the motor by using semiconductor switches connected to an outside of the motor (see [Abstract] and FIG. 1). Use of the semiconductor switches enables switching while the motor is being driven.
Patent Document 3 describes a method in which two inverter main circuits are connected to motor windings instead of forming a neutral point of the motor windings, and the driving phases of the two inverter main circuits are adjusted, so that a voltage to be applied to the motor can be increased from a normal inverter drive level (see [Abstract] and FIG. 1). This method is also capable of enlarging the drive range without stopping the motor.
Patent Document 4 describes a method in which a pair of inverter main circuits (two inverter main circuits) are connected to windings, respectively, and at least one of the back electromotive force waveform and the current waveform of each winding is formed into a pseudo-rectangular wave shape (see [Abstract] and FIG. 7). This method is also capable of enlarging the drive range without stopping the motor.
Patent Document 5 describes a method in which plural inverter main circuits are connected to terminals of motor windings connected in series, and the motor is driven at a low-speed rotation by use of the inverter main circuit connected to the windings connected in series, and driven at a high speed rotation by use of the inverter main circuit connected to intermediate terminals of the windings connected in series (see [Abstract] and FIG. 1). This method involves driving by switching the inverter main circuits depending on the driving condition of the motor. Thus, this method is capable of switching the windings without stopping the motor, and causes no loss in the inverter main circuits or the like, since the number of inverter main circuits driven is always only one.
The foregoing Patent Documents 1 to 5 have the following problems, however.
The technique disclosed in Patent Document 1 needs a mechanical change of the mechanical switch or the like, and requires the motor being driven to be stopped once to switch the mechanical switch. For this reason, this method is difficult to apply no apparatuses which need to be driven continuously, such as main engines of electric automobiles.
The technique disclosed in Patent Document 2 is the method of switching by use of semiconductor switches, and is capable of switching while the motor is being driven. However, this method has a problem that a loss in the semiconductor switches always occurs.
The techniques disclosed in Patent Documents 3 and 4 require the two inverter main circuits for driving the motor to be driven concurrently, which leads to a problem that a loss in the inverter main circuits is doubled to reduce the circuit efficiency.
The technique disclosed in Patent Document 5 solves the aforementioned problems pointed out regarding Patent Documents 1 to 4, but still has room for improvement in achievement of higher efficiency at a low-speed rotation range and enlargement of the drive range at a high-speed rotation range, because the motor itself has a conventional structure.